As I was preparing my slides for my third year undergraduate stat course, I got a weird error that got a search on the Web to unravel:

! Extra }, or forgotten endgroup.
endframe ->egroup
  begingroup def @currenvir {frame}
l.23 end{frame}
  begin{slide}
?

which was related with a fragile environment

begin{frame}[fragile]
frametitle{simulation in practice}
begin{itemize}
item For a given distribution $F$, call the corresponding 
pseudo-random generator in an arbitrary computer language
begin{verbatim}
> x=rnorm(10)
> x
 [1] -0.021573 -1.134735  1.359812 -0.887579
 [7] -0.749418  0.506298  0.835791  0.472144
end{verbatim}
item use the sample as a statistician would
begin{verbatim}
> mean(x)
[1] 0.004892123
> var(x)
[1] 0.8034657
end{verbatim}
to approximate quantities related with $F$
end{itemize}
end{frame}begin{frame}

but not directly the verbatim part: the reason for the bug was that the end{frame} command did not have a line by itself! Which is one rare occurrence where the carriage return has an impact in LaTeX, as far as I know… (The same bug appears when there is an indentation at the beginning of the line. Weird!) [Another annoying feature is wordpress turning > into > in the sourcecode environment…]